With the advent of modern telecommunications technology, telecommunications services are being offered in more and more remote regions of the world. Telecommunications services have been offered to these remote areas in a variety of ways. For instance, several satellite based systems have been proposed to enable direct communication between handsets and the satellite even when the mobile station is located in a region of the world far removed from wire-based or cellular telephone services. As proposed, the foregoing satellite systems include individual user handsets which afford bidirectional communication directly with the telecommunications satellite. The handsets do not communicate directly with any other handset. The term "handset" as used throughout includes mobile stations, portable phones, cellular phones, wire-based stationary phones, faxes, modems and the like.
However, the current handset-to-satellite link is considered to have certain limitations, particularly with respect to signal blockage. In order to effect direct communication between the handset and satellite, the communications link requires an unobstructed line of sight therebetween or an "open sky" void of obstructions, such as buildings, trees and the like. Such intervening structures interfere with the communications link. To minimize the risk of obstructions between the satellite and handset, the handsets should be used in open areas away from large buildings. Alternatively, the handsets may be attached to stationary antennas, such as a rooftop omni or dish-type antenna. However, such connections greatly constrain the portability of the handsets, thereby rendering the handset a stationary unit. Stationary units may have limited use, such as in regions which do not have existing telephone infrastructures or in areas where the existing telephone system is prohibitively expensive and/or unreliable. However, a telephone system consisting of stationary units would be less economical and require more satellite links than the proposed invention.
Moreover, notwithstanding the use of stationary roof mounted antennas, the stationary units heretofore proposed still experience limitations, due to the use of an asynchronous communications uplink. More specifically, the telecommunications satellite systems thus far proposed contemplate using a synchronous down link when the satellite transmits data to the mobile station. However, each satellite must receive uplink communications data from a large number of individual mobile stations. Some proposed systems (Irridium and Global Star) contemplate using asynchronous uplinks. Another proposed system (Odyssey, proposed by the assignee of the present application) contemplates using synchronous uplinks. The concept of synchronous and non-synchronous communications are explained in more detail in the book entitled "Satellite Communications Systems" by Maral et al., published by John Wiley & Sons, New York, N.Y. (1986), and in the book entitled "An Introduction to GSM" by Redl et al., published by Artech House Publishers, Boston, Mass. (1995). Both of the above-referenced books are expressly incorporated by reference in their entirety. Synchronous communications are achieved based on timing information provided to all mobile stations. The timing information is provided separately to each mobile station. The timing information controls advancements and retardation of the time at which a mobile station begins transmission to ensure that the data is received synchronously from all transmitting mobile stations. Timing information is generally used in connection with synchronous return links from mobile stations.
When using non-synchronous uplinks, the system greatly limits the capacity of the uplink as compared to a synchronous uplink. Non-synchronous direct sequence spread spectrum communications links experience substantially greater co-channel interference as compared to synchronous links. To minimize the co-channel interference, the system typically employs substantial forward error correction encoding and limits the number of mobile stations supported by the satellite, thereby decreasing the system's capacity and economical viability. When using synchronous uplinks, the proposed systems (Odyssey) require complex circuitry to be installed in each mobile station to control timing advancements and retardations and individual handset output power. This synchronization and power control circuitry increases the complexity and cost of each handset.
In addition, portable handsets have a limited battery dependent power supply, thereby limiting the maximum achievable output power level and duration of portable service. Satellite based telecommunications systems, such as the Odyssey system, contemplate using low power generally omni-directional antennas with the mobile stations. Such omni-directional antennas transmit outgoing communications typically with more or less the same power in all directions. These omni-directional or hemisphere coverage antennas may operate at power levels as low as one-half watt. These power constraints further render the communications link with the handset susceptible to line of sight obstructions between the handset and satellite.
Moreover, the proposed satellite based telecommunications systems require unduly excessive communications traffic with the satellite. Generally, communications links in satellite based systems may be categorized into (1) local calls between two local handsets located within a relatively small geographic area, and (2) long distance calls between one local handset and a remote destination handset. The terms "local" and "remote" as used throughout this application are used relative to a predefined geographic area or cell surrounding the handset initiating the call. If the destination handset falls within this geographic area, the call is deemed local. Otherwise, the call is deemed remote or long distance.
The proposed satellite based systems route all outgoing calls from an initiating handset through a corresponding satellite located overhead the initiating handset. The satellite then transmits each call to a network ground or earth station which decodes the call destination. If the destination is also a PCS satellite handset, it determines which satellite and ground station connections are necessary to direct the call to the destination handset. The network ground station is connected to a PSTN network. If the destination handset is also local, the network ground station transmits the signal back to the overhead satellite above the predefined geographic area surrounding the initiating handset. The satellite transmits the call to the destination handset. If the destination handset is remote, the network ground station may transmit the call via wireline network and a ground station and/or satellite. Thus, all calls must be routed through the overhead satellite and network ground station, regardless of whether the destination handset is local or remote. Such routing unnecessarily increases the traffic which must be supported by the satellite and network ground stations.
Conventional systems have failed to address the deficiencies noted above within the field of satellite based telecommunications. Examples of existing systems are disclosed in U.S. Pat. No. 5,173,933 and U.S. Pat. No. 5,412,660 both of which are expressly incorporated by reference in their entirety.
The '660 patent discloses a satellite based ISDN network. The network includes a plurality of portable satellite terminals which receive and transmit communications which conform to the ISDN industry standard and which operate with standard ISDN equipment. The network of the '660 patent extends the public service telephone network (PSTN) access to isolated single terminals. However, the '660 patent is not concerned with extending PSTN access to a remote community with multiple users therein, and hence is not concerned with providing remote intra-community communications paths without accessing overhead satellites. Instead, the system of the '660 patent transmits all ISDN data via the satellite without distinguishing between remote and local destinations.
The '933 patent discloses an interface between mobile telecommunications stations and trunk channels linked to various communications carriers, such as a satellite dish, a cellular line antenna and the like. The interface automatically selects the most cost-effective carrier upon which to transmit an outgoing telephone or fax transmission from a mobile station. However, the interface of the '933 patent does not distinguish between local and remote destinations. Nor does the interface of the '933 patent bypass the satellite or antenna for entirely local calls. Instead, the interface of the '933 patent is intended solely for remote calls, namely calls which must be relayed through a satellite or similar communications carrier. The '933 patent is intended for implementation upon an oceangoing vessel and is intended to provide a communications link between callers on the vessel and parties on shore. The communications system of the '933 patent is not concerned with intra-ship communications. Thus, the interface of the '933 patent does not provide a local community with a low cost telecommunications infrastructure.
A need remains for an improved satellite based telecommunications system for use in a remote community that can economically interface with or provide the basis for a wired or wireless/cellular-type telecommunications infrastructure. It is an object of the present invention to meet this need.